Evidence from patients with Alzheimer's disease suggests that the basal forebrain cholinergic system plays an important role in memory functioning. In support of this view, it was found that lesions of the basal forebrain in monkeys produce impairment in recognition memory and altered sensitivity to the effects of drugs that act on memory. Our results in mormal monkeys show that compounds that interfere with cholinergic mechanisms, such as the anticholinergic agent scopolamine, produce impairments in recognition memory, wherea thosee compounds that enhance cholinergic activity, such as physostigmine, improve recognition memory. In addition, our results suggest that the anterograde effects of scopolamine on memory may be greater than its retrograde effects, implying an influence on storage rather than retrieval. In a separate study, we found that THC, the active ingredient of marijuana, impairs recognition memory but is without effect on habit formation even in high doses, a dissociation implicating the limbic system as the site of action of this drug.